A woman in her 70s was admitted to our institution with complaints of right hypochondrium pain. Abdominal computed tomography revealed a 13-mm retroperitoneal tumor between the liver and right kidney. The tumor rapidly increased to 82mm within 2 months, a necrotic change was inside the tumor, and the inflammation spread to the surrounding diaphragm and the peritoneum. The patient underwent surgical resection including the affected diaphragm and the peritoneum. Histopathological examination revealed a myofibroblastic spindle-cell proliferation with prominent infiltration of inflammatory cells, such as the plasma cells, lymphocytes, neutrophils, and eosinophils, diagnosed as an inflammatory myofibroblastic tumor (IMT) based on positive smooth muscle actin staining. IMT arising from the retroperitoneum is a rare case in Japan;we report this case with literature review.